An object detection apparatus that detects an object by transmitting and receiving an ultrasonic wave includes an ultrasonic sensor. The device measures the time interval from when the ultrasonic sensor transmits an ultrasonic wave to when a reflection wave of this ultrasonic wave reflected by an object is received, to determine the distance to the object.
When there is another ultrasonic sensor around the object detection apparatus, crosstalk may occur. Crosstalk means reception, by the ultrasonic sensor of one's device, of a wave transmitted by the ultrasonic sensor of another device. It is difficult to distinguish the wave transmitted from the ultrasonic sensor of the other device from a reflection wave of the transmission wave from the ultrasonic sensor of one's device. Therefore, when crosstalk occurs, it causes an error in the calculated distance.
Moreover, in a driver assistance system where a driver assistance function is executed based on the distance detected by this object detection apparatus, such error in the distance may cause a malfunction of the driver assistance system.
In Patent Literature 1, to prevent crosstalk, an infrared sensor that transmits and receives an infrared beam is further provided, so as to synchronize the timing of transmitting an ultrasonic wave with that of other ultrasonic sensors.
However, providing an infrared sensor for preventing crosstalk causes a cost increase. In addition, not all the devices are necessarily equipped with an infrared sensor of the same specification, and it is impractical to add an infrared sensor to the devices that are already operating in the field. To prevent crosstalk, the interval of transmitting an ultrasonic wave may be varied randomly. The time or time lag in receiving the reflection wave from an object is not affected by the variation in the transmission interval.
On the other hand, varying the time of transmitting a wave from the ultrasonic sensor of one's device randomly will cause the time lag in receiving a transmission wave from the ultrasonic sensor of the other device to vary randomly. This is because the timing at which the ultrasonic sensor of the other device transmits a wave is not affected by the interval at which the ultrasonic sensor of one's device transmits a wave. Therefore, by varying the interval of transmitting an ultrasonic wave randomly, crosstalk can be distinguished.
For the crosstalk between object detection apparatuses of the same type, the transmission interval of the ultrasonic sensor in the other object detection apparatus is also varied randomly. Even though the ultrasonic sensor of one's device varies its transmission interval, if the ultrasonic sensor of the other device also changes the transmission interval and this changed interval happens to be the same as that of the one's device, the crosstalk cannot be distinguished. In order to minimize the possibility that these transmission intervals match, there needs to be as many transmission intervals as possible. With a large number of transmission intervals, however, the longest transmission interval will become longer, as compared to when there are fewer transmission intervals. This is because the longest transmission interval cannot be made shorter than a length determined by the device's minimum processing cycle multiplied by the number of transmission intervals provided. Selecting one from this large number of transmission intervals randomly causes the transmission/reception cycle to be long.